As one of the basic analysis parameters used in various fields related to the environment, food, medicine, pharmaceuticals, etc., pH is used, and to measure this, a pH sensor is used.
As a conventional pH sensor, a pH-selective glass membrane electrode has been generally used. However, the electrode is fragile, and difficult to be activated when fouled.
That is, for examples, as a water quality monitoring system is operated for a long period, the sensor is exposed to an environmental sample for long period, or severe fouling on a surface of the sensor is caused due to the properties of the samples in soil analysis, a food process, etc. Consequently, as well as deactivation of the pH sensor caused thereby, severe fouling is caused. For this reason, the sensor has to be periodically replaced or activated.
Metal oxide films formed on the surface of the metal electrodes may be used to measure the concentration of hydrogen ions because the redox potentials of the oxides are dependent on the concentration of hydrogen ions. Such metal oxides may be TiO2, RuO2, RhO2, SnO2, Ta2O5, OsO2, PdO2, PtO2 or IrO2 (Non-patent document 1).
In the hydrogen ion-selective electrodes using metal oxides, reversible redox potentials of the metal oxides are dependent on the concentration of hydrogen ions. In the manufacturing of the metal oxide film pH electrodes, sputtering, high-temperature thermal decomposition or electrochemical oxidation of a metal wire are applied.
A typical electrochemical technique is an oxidizing method of iridium (Ir electrode to an iridium (IV) oxide, for example, by dipping an iridium (Ir) metal electrode in a sulfuric acid solution, and cycling the potential of the electrode between −0.25 and +1.25 V (vs. SCE) repeatedly. The iridium oxide formed on the surface of the metal electrode is known to be a hydrated iridium oxide in a form of IrO2.4H2O, Ir(OH)4.2H2O, and [IrO2(OH)2.2H2O]2−, and to have a super-Nernstian response of about −90 mV/pH unit (Non-patent document 2).
Also, in the case of iridium oxide film electrode prepared on the surface of platinum or iridium, or a conductive metal electrode by sputtering or thermal decomposition, the iridium oxide on its surface is known to be an anhydrous iridium oxide, and exhibits a sensitivity of about −59 mV/pH with respect to the concentration of hydrogen ions (Non-patent document 3).
Since the iridium oxide coated film is formed at high temperature or in a high vacuum environment, the iridium oxide coated film electrode manufactured by sputtering or thermal decomposition has microscopic uniformity of the coated film and micropores formed during the thermal decomposition, and therefore a permeation time of a solution is increased, resulting in a delayed sensing rate.
Also, when a measuring solution is changed, prompt and complete change of the solution in the coated film is difficult. Thus, there are problems of the delayed sensing time, low reproducibility, and a sensitivity slope deviating far from the theoretical sensitivity slope (−59.2 mV/pH). Also, when its surface is fouled, or the sensitivity of the electrode is degraded, it is not easy to regenerate the electrode properties effectively, resulting in a reduced lifetime of the electrode.
Recently, a modified glass or ceramic composite material electrode manufactured by mixing a conductive metal microparticle mixture with a glass or ceramic powder, molding the mixed product, and sintering the molded product at high temperature was reported (Non-patent document 4 and Patent document 1).
However, according to the research conducted by the inventors, the above-described technique has problems of low reproducibility of the electrode signal, low sensing rate due to the time needed for the surface equilibration, and a high hysteresis with respect to abrupt pH change, and since the electrode is manufactured by mixing, molding, and then sintering the glass or ceramic powder at high temperature, the electrode is difficult to be manufactured.
Meanwhile, a hydrogen ion electrode in which carbon black and iridium oxide particles were bound using polymer resin binder was reported (Non-patent document 5).
However, according to the research result by the inventors, although the hydrogen ion electrode has excellent physical stability and surface renewability, compared to the conventional glass electrode or polymer membrane electrode, the manufacturing process is complicated because it is necessary to add a conductor such as carbon black. Also, the pH dependency of the polymer electrode has a large variation in a range from 50 to 60 mV/pH by production lot. Also, a relatively high hysteresis of about 5 mV is observed when the pH is changed abruptly in wide range, and measured pHs have large errors of about 0.1 pH unit.